(634c) Preventing Coarsening of Foams Using Hydrophobically Modified Silica Nanoparticles

Aqueous foams are familiar complex fluids of bubbles in a continuous water phase, and ensuring their stability is essential in applications in the food, pharmaceutical and personal care industries. Foams breakdown by coalescence (bubble merging) and coarsening (gas transport from small to large bubbles) and have traditionally been stabilized by surfactants in the aqueous phase. Surfactants adsorb to the bubble interfaces and provide barriers to coalescence but are ineffective in preventing coarsening as they desorb with bubble shrinkage providing only a small resistance to compression. Hydrophilic nanoparticles (NPs) in water also adsorb to air/water interfaces when the NP surface is hydrophobically modified. NPs adsorb irreversibly and at high enough bulk concentration and surface hydrophobicity they form closely packed interfacial layers which resist compression as the interfacial area shrinks during coarsening. This property, along with the fact they can potentially be easily resourced, sustainable and green (e.g. nanocellulose crystals) make them ideal alternatives to surfactants for foam stabilization.

A model system for studying the properties of NP interfacial layers for foam stabilization are negatively charged colloidal silica (20 nm) whose surfaces are hydrophobically modified by the adsorption of a positively charged surfactant, cetyltrimethylammonium bromide, CTAB. The ratio of CTAB (mM) to particle concentration (wt%) defines the degree of hydrophobicity. We focus on coarsening and using microfluidics to study coarsening in a two dimensional foam, we identify NP concentrations and hydrophobicities most effective at preventing coarsening. These measurements are correlated with measurements of the dilatational viscosity of the adsorbed interfacial layers to demonstrate that a high enough elasticity relative to the tension is necessary to prevent coarsening (Gibbs-Laplace criteria). X-ray reflectivity (XRR) and grazing incidence small angle x-ray scattering (GISAXS) experiments are undertaken to elucidate the nanoscale structure of the interfacial layers and correlate the structure with the elasticity. These measurements reveal that only a single layer of NPs adsorb in a hexagonal array, and high elasticities can be achieved without close packing. High elasticity may originate from CTAB molecules adsorbed directly on the interface and between the particles, which indicates a synergism between surfactants and particles in preventing coarsening.